The book covers biological problems of the freshwater environment. The nature of the problems, the importance of surface water resources, the threats of pollution damage from progressive expansion in population and industrialization are discussed. An extraordinary value prevails for water resources as a recreation medium in addition to the vital domestic water supply and industrial supply functions. The procedures utilized to survey various water bodies and evaluate their changes with pollution stresses are cited in considerable detail. The aquatic environment receives human wastes and thereby becomes a potential medium for spread of infectious diseases. The indicator organism studies used for evaluation of this problem is covered. A great variety of illustrations, references and tabulated data are provided.